Water softeners have been sold for years as a solution to scaly rings in the bathtub, deposits in the water heater and anemic dishwater suds, generally without exaggerated health claims.

Water softeners are “ion exchangers”.  In the softener, hard water flows through a tank filled with synthetic resin beads.  Millions of sodium ions are loosely attached to each bead.  The passing water exchanges its "hardness" ions, mostly calcium and magnesium, for the "soft" sodium ions on the beads and then flows on to the faucets.  When the resin becomes saturated with hardness ions, it must be “regenerated” in order to work again.  During regeneration, salt water from a brine tank flows through the resin and it exchanges hardness ions for sodium ions.  During the backwash cycle of regeneration, accumulated hardness ions and excess brine go down the drain.

Water's hardness Is usually expressed as grains per gallon (gpg), equivalent to the amount of calcium carbonate in the water.  According to Industry benchmarks:

A softener is designed to remove calcium and magnesium the minerals that lead to soap-curd deposits in the bathtub and sinks, dull-looking laundry, spots on dishes, scaly deposits on faucets and shower heads, scale inside the water heater and pipes.

If you decide a water softener is what you need, shop around.  Contact two or three dealers to find out what equipment they recommend and what it will cost to have it installed.  Prices vary dramatically, depending on where you live and the complexity of your water problem.

There's no reason to choose one brand or model over another on the basis of softening performance.  A dealer can tailor his unit’s softening capacity to the hardness of your water and your pattern of water consumption.  Choosing a softener, then, is largely a matter of selecting a unit that’s easy to maintain and handy to use.

A water softener shouldn't require much care and feeding beyond the periodic addition of salt.  Be sure the softener you choose has an opening that’s large enough and low enough to make refills easy. 

A matter of control
Assume any water softener will do an acceptable job of removing minerals. The simplest and cheapest softeners run on a timer that regenerates on a fixed schedule.  That may be fine if your daily water use is reasonably predictable.  Overall, however, timer-only units tend to use salt and water inefficiently.  Demand-control softeners can be set to regenerate in line with your pattern of water use.  That means the softener will use less salt and waste less water.  The fanciest controls, on top of-the-line units, may be more than you need.  Controls on less expensive units work just as well and are less daunting to adjust or reset.